Conventional vacuum cleaners are operated with a drive unit which is controlled in an open-loop manner. The drive unit is generally the motor of a suction fan. An input power is set for the drive unit or motor, either as a fixed power level or one which is variable by the user via a selector, and is controlled via a voltage level, for example, by means of phase control. The set input power is then obtained for a specific operating point on the flow rate-vacuum curve, and it changes when the vacuum or flow rate changes because of operating conditions. The reasons for this may be different types of floor covering or a reduced throughput of air when the separator device, in particular, a dust bag, is full.
In order to prevent the flow rate from decreasing when the dust bag is full, it some vacuum cleaners include a flow rate control system. In the case of very dense floor coverings, the throughput of air decreases rapidly and, therefore, the flow rate is very difficult to control at the desired level.
German Patent Application DE 10 2007 057 589 A1 describes a flow rate control system with superimposed push force control for a vacuum cleaner. The control unit alternatively accesses a first pressure sensor in the area of the floor nozzle and a second pressure sensor between the dust bag receptacle and the motor.
German Patent Document DE 10 2008 010 068 describes a vacuum cleaner where the suction power at the suction nozzle is intended to be maintained approximately constant. To this end, two pressure sensors are used to provide measured values for two-stage pressure control.
Two pressure sensors are also used in a vacuum cleaner described in U.S. Pat. No. 5,881,430.
While it is indeed important for the user of a vacuum cleaner to limit the force required to push the floor nozzle, it may be more important that the suction power at the vacuum attachment remain as constant as possible. In order to ensure that sufficient suction power can be provided even when the dust bag is filled to its maximum capacity, a high power input must be selected for the drive unit. However, when the dust bag is empty, this high input power is generally excessive and counterproductive, because it operates at high vacuum levels and has corresponding disadvantages in terms of energy consumption.